A 35-foot Doppler radar dome showing the back of the 28-foot radar dish. The dish spins around the pedestal in lower left. (Contributed -- NOAA/NWS)

SANTA CRUZ >> On the shoulder of Mount Umunhum in the Santa Cruz Mountains is something that, from afar, resembles a golf ball on a stick. Get closer, however, and it begins looking more like a giant soccer ball.

But it’s no plaything. Inside spins the National Weather Service’s main Doppler radar for the greater Bay Area.

Many local weather experts, though, say it’s simply in the wrong location. And how it got there is a two-decades-old tale of local weather politics that still affects Bay Area forecasts to this day.

“If you were to pick the worst spot to put a radar in the Bay Area, it probably would have been in the Santa Cruz Mountains,” said Jan Null, a meteorologist who owns Golden Gate Weather Services in Saratoga.

Null and others say Mount Umunhum, known to locals as “Mount Um,” is too far south to see big winter storms approaching from Alaska. Mount Umunhum is about 30 miles northeast of Santa Cruz.

“The Pacific Ocean is still a blind spot,” said veteran meteorologist Rob Mayeda of San Jose-based KNTV.

Doppler radar can see rain, snow and hail for hundreds of miles by bouncing radio waves off water and ice in storm clouds. The antenna can point at different angles upward, but not down. But at 3,535 feet above sea level, the Mount Um radar is too high. It can’t see rain or severe weather below 3,600 feet.

Null said that’s a big problem because the bottom of most rain clouds in the Bay Area ranges from 1,500 to 2,500 feet.

It gets worse. Because of the curvature of the earth, the radar beams rise higher the farther away they are from Mount Um.

Over San Francisco, the radar can’t see anything below 5,000 feet. Soon after the radar was installed in 1994, San Francisco set a new 24-hour rainfall record of 6.16 inches. The new radar recorded less than three-quarters of an inch.

So why is the radar in such a bad spot?

Site saves money

In the late 1980s, the Weather Service wanted to move its Bay Area office out of Redwood City to save money. It considered several locations, including San Jose State and San Francisco State.

Then the Navy offered “free” space in Monterey near its worldwide meteorology teaching, research and forecast centers. The Weather Service also liked the potential for cooperation with the Navy, said Null, who was working for the service at the time.

Over the protests of local forecasters, who worried about losing 200 years of collective experience with San Francisco Bay Area microclimates, the Weather Service decided to move to Monterey.

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About the same time, the service was rolling out its new “Nexrad” Doppler radars nationwide. And since the old radar in Sacramento didn’t cover the Central Coast, the service planned to place one of the new radars in the Bay Area.

Close to Monterey office

The Bay Area radar needed to be on a mountain near the newly chosen Monterey office, so the Weather Service chose Mount Um. It was close enough to Monterey for maintenance and a low-cost microwave link to send the radar data.

Null said the Navy’s promises didn’t work out because the Navy space wasn’t free after all and the government agencies were just too different to effectively cooperate. In addition, he said, the microwave signal proved to be unreliable, so the Weather Service had to install an expensive high-speed telephone line anyway.

Another problem: The Mount Um radar leaves holes in coverage for the North Bay — holes big enough to miss some “atmospheric river” storms. So two local television stations were forced to take matters into their own hands.

KGO and KPIX installed high-definition Doppler radars in the North Bay. But both are too high.

“We pick up a lot of virga, which is rain that actually evaporates before it hits the land,” said KPIX forecaster Roberta Gonzales, who has studied Bay Area weather for 20 years. She often wonders: “Is that rain I’m seeing in the North Bay right now, or is that virga?”

Yet another problem: Mount Um’s radar can be hard to maintain and repair.

If something breaks, the Weather Service sends a two-person crew from Monterey to fix it. In good weather and light traffic, that’s a 90-minute drive. Accidents, fallen trees and mudslides make the trip a lot longer.

“Access can be part of the problem, especially in strong winter storms,” said Logan Johnson, the warning coordination meteorologist at the Weather Service’s Monterey office.

When it rains

So if it’s raining cats and dogs and the Mount Um radar is down, forecasters fall back on other tools.

Automated rain gauges indicate how fast the rain is falling in selected locations. Weather satellites reveal approaching storm clouds. Forecasters even use eyewitness reports of downpours, hail and high winds.

With all these other tools available, some government forecasters say, the Mount Um radar location really isn’t all that bad. “You have to look at all the data sources available,” Johnson said.

And there’s some good news for the 20-year-old radar: The Weather Service has been continuously upgrading and maintaining its Doppler radars.

“We do a really rigorous preventive maintenance schedule on it,” Johnson said. “We want to prevent it from actually breaking during hazardous weather.”

Now wireless

The Weather Service also added a wireless connection to get data from the Mount Um radar when the main phone line goes down.

In 2012, the agency replaced several aging components. “Dual polarization” Doppler radar can now see the difference between small raindrops, large raindrops, hail and even snow.

KNTV’s Mayeda compared it to the aftermath of a trip to the eye doctor.

“It’s kind of like having blurry vision and now suddenly you’ve got a real sharp set of glasses on,” Mayeda said.

But critics point out that the upgrades do little to fix the basic problem: location.

Null of Golden Gate Weather and others think the Weather Service could have chosen a much better place than Mount Um — low enough to see most of the rain, with good views of approaching Alaskan storms, easy access and better coverage for millions of people.

“The ideal spot probably would have been Mount Tamalpais” in Marin County, Null said.